1. Field of Invention
The present invention relates to power-line-operated electronic inverter-type fluorescent lamp ballasts.
2. Background Considerations
In an ordinary non-instant-start-type fluorescent lamp, a cathode must be heated to incandescence before electron emission starts and the lamp becomes conductive; and this heating process is apt to require from one to two seconds. Thus, aside from the relatively modest amount of power needed to accomplish the conditioning, and provided that the lamp is not driven into a destructive instant-start mode, a fluorescent lamp is substantially a non-conducting load until its cathode has reached incandescence.
In many applications of electronic inverter-type ballasts for fluorescent lamps, it is often desirable to power the lamp by way of having it parallel-connected across the tank capacitor of a high-Q resonant L-C circuit--with this L-C circuit itself being series-connected directly across the inverter's output.
However, when such a high-Q series-resonant L-C circuit is not loaded, it acts substantially as a short circuit; which, during even a brief period (such as during the one or two seconds it takes for the lamp to become effectively conductive), is apt to cause destructive overload of the inverter and/or the L-C circuit itself.
One way of preventing such destructive overload is that of connecting in parallel with the lamp a voltage-limiting means (like a Varistor) characterized by: (i) not conducting at the highest magnitude of voltage normally present across the lamp when it is conducting; and (ii) conducting heavily at a voltage of somewhat higher magnitude than that.
However, in many applications, due to the significant amount of energy that must be absorbed by this voltage limiting means, even if the required conditioning period is only a couple of seconds in duration, the effective cost associated with such a method of preventing destructive overload of inverter and/or L-C circuit is very high.
Also, the potential inefficiency involved--as for instance in a situation where the lamp fails to ignite or is simply removed from the L-C circuit--can represent a major obstacle to designing a fully functional product.